1. Field of the Invention
This invention in general relates to positive temperature coefficient of resistance (PTC) conductive polymer compositions and electrical devices containing such compositions, and, more particularly, to PTC conductive polymer blend compositions comprising at least two types of polymers and preferably at least two types of carbon blacks, and to electrical devices containing such compositions.
2. Description of the Prior Art
It is well-known that polymers can be made electrically conductive by dispersing therein suitable amounts of conductive particulate fillers such as carbon black or fine metal particles. Some of the conductive polymers exhibit PTC (positive temperature coefficient of resistance) behavior, and have been used in various electrical devices such as resettable circuit protection devices, self-regulatory heaters, and temperature and current sensors. Simply speaking, a PTC conductive polymer composition is characterized by its behavior that upon approaching a critical temperature (i.e., the switching temperature or Ts), a sudden increase in resistance takes place and, as a result, the current flow through the polymer is greatly reduced.
For a number of reasons, PTC conductive polymers have been gradually replacing conventional ceramic based PTC materials, e.g., BaTiO3-doped ceramic thermistors, in many applications. First, conductive polymer composites can be made such that their resistivity is much less than 7; ohm-cm, and sometimes as low as 1 ohm-cm, which is approximately one order of magnitude lower than that of ceramics. This means that for the same electrical device dimensions the resistance is approximately ten times smaller. This low-resistance feature allows a higher hold current (i.e., larger steady-state running current) to pass though the electrical device without causing the device to heat up and latch or xe2x80x9ctripxe2x80x9d into the high-resistance state. Second, PTC polymers generally have better manufacturability than ceramics. Conventional thermoplastic processing methods such as melt mixing, extrusion, injection molding, continuous lamination, and spin-coating can all be used for preparing conductive polymer composites.
Although superior to ceramic PTC materials in many respects, prior-art PTC conductive polymer materials do have a number of disadvantages. The first of such disadvantages is that PTC conductive polymer composites generally exhibit the so-called resistance hysteresis effect, i.e., their resistance after heating or tripping does not immediately return to the initial value upon cooling, resulting in elevated power dissipation after resetting. This feature can have a severe negative effect on an electrical device (e.g., a resettable fuse) containing such composites if the device is to form part of a matched resistance circuit.
One of the approaches in the prior art to minimize the resistance hysteresis effect is to use a mixture of carbon blacks differing in their particle sizes. For example, U.S. Pat. No. 5,705,555, entitled xe2x80x9cConductive Polymer Compositions,xe2x80x9d discloses a conductive polymer composition comprising a mixture of two types of carbon blacks, each of which constitutes from 1 to 40 percent by weight of the composition and has a structure level (or pore volume), as measured by the DBP absorption technique, of 40 to 150 cc/100 g. The two types of carbon blacks differ only in their average particle sizes, i.e., one of them has an average size in the range from 35 to 300 nm and the other has an average size in the range from 15 to 25 nm.
Another approach in the prior art to reduce the resistance hysteresis effect and improve resistance stability is to use a blend of two polymers with different melting points. For example, U.S. Pat. No. 5,451,919, entitled xe2x80x9cElectrical Device Comprising a Conductive Polymer Composition,xe2x80x9d discloses a PTC conductive polymer composition comprising two crystalline fluorinated polymers the melting points of which differ by 25 xc2x0 to 100xc2x0 C.
Still another prior-art practice of making an electrical device containing a cross-linked PTC tip conductive polymer composition is taught in U.S. Pat. No. 4,560,498, entitled, xe2x80x9cPositive Temperature Coefficient of Resistance Compositions,xe2x80x9d which includes the use of a first polymeric material exhibiting high green strength prior to cross-linking and elastomeric behavior after cross-linking, and a second polymeric material comprising a thermoplastic, both of such polymeric materials having dispersed therein conductive particles such as carbon black.
Yet another prior-art practice of making an electrical device containing a PTC element, as taught in U.S. Pat. No. 5,554,679, entitled xe2x80x9cPTC Conductive Polymer Compositions Containing High Molecular Weight Polymer Materials,xe2x80x9d is to use a melt-extrudable polyolefin matrix, in which a volume-expansion-regulating, high-molecular-weight polymer and a conductive particulate filler are dispersed. Additionally, the volume-expansion-regulating polymer should have minimal migration upon heating of the composition and should not exceed 50% by weight of the total polymeric portion such that interference with the extrudability of the polyolefin matrix would not occur.
A second disadvantage of PTC conductive polymer composites is that, to make PTC materials having a sufficiently low resistivity, the conductive filler content generally has to be greater than approximately 35 volume percent. Such high filler loading generally results in elevated viscosities, causing various processing difficulties during the preparation of the devices using the polymer, particularly during melt extrusion of conductive polymer sheets and lamination of metal electrodes onto the conductive polymer substrate. High filler loading may reduce the bonding strength between an electrode and the conductive polymer, which in turn would cause foil delamination and high contact resistance after the exposure of the device to high ambient temperatures.
To alleviate the aforesaid contact resistance problem associated with poor bonding, most prior-art practices involve the use of a specially treated or roughened surface to improve its adhesion to the conductive polymer. Typically, such specially treated surfaces have a center-line average roughness Ra of at least 1.3 as measured by using a profilometer with a stylus of 5 xcexcm radius. Alternatively, special processing schemes have been utilized. For example, U.S. Pat. No. 4,876,440, entitled xe2x80x9cElectrical Devices Comprising Conductive Polymer Compositions,xe2x80x9d teaches the formation of an electrode-to-polymer interface by contacting the molten polymer composition with the electrode while the electrode is at a temperature above the melting point of the polymer composition.
It is therefore an object of the present invention to provide a PTC conductive polymer composition having a reduced resistance hysteresis.
It is another object of the present invention to provide an electrical device comprising electrodes and a PTC conductive polymer composite, which device has reduced contact resistance between the electrodes and the polymer composite.
It is yet another object of the present invention to provide a PTC conductive polymer blend composite comprising two or more types of polymers which can be selected based on an easily ascertainable physical, functional or dynamic property of the polymers.
It is a further object of the present invention to provide a PTC conductive polymer composite air containing two or more types of conductive particulate materials (e.g., carbon blacks) which can be selected based on easily ascertainable physical, functional, dynamic or kinetic properties of the materials.
Briefly, according to one aspect of the present invention, the composition of a conductive polymer blend composite suitable for use in electrical devices requiring the PTC characteristic can be predetermined by selecting two polymeric materials, each having a melt flow index (MFI) different from the other. These two polymer materials can have either similar or dissimilar molecular structures. As used herein, polymers having similar molecular structures, such as two types of high-density polyethylene (HDPE), are homologous polymers, whereas polymers having dissimilar molecular structures, such as a polyethylene and a copolymer of ethylene and vinyl acetate, are nonhomologous polymers.
Preferably, the composite of the present invention further comprises two types of conductive particulate materials, each having an average size and a DBP value different from the other.
In a PTC conductive polymer blend composite of the present invention, in case the two polymeric materials are homologous, one of the polymeric materials should have an MFT value approximately 3 to 15 and constitute approximately 5 to 45 percent by weight of the composite, while the other polymeric material should have an MFI value less than approximately 1 and constitute approximately 2 to 40 percent by weight of the composite; alternatively, in case the two polymeric materials are nonhomologous, one of the polymeric materials should have an MFI value of approximately 5 to 15 and constitute approximately 5 to 45 percent by weight of the composite, while the other polymeric material should have an MFI value less than approximately 1 and constitute approximately 2 to 40 percent by weight of the composite. In either case, if two types of conductive particulate materials are used, one of them should have an average particle size of approximately 60 to 120 nm and a DBP value of approximately 60 to 200 cc/100 g and should constitute approximately 45 to 56 percent by weight of the composite, while the other conductive particulate material should have an average particle size of approximately 10 to 20 nm and a DBP value of approximately 250 to 500 cc/100 g and should constitute approximately 2 to 10 percent by weight of the composite. It is understood that the total percentages of all the constituents or components of any composite above do not exceed 100 percent.
In another aspect the present invention provides a conductive polymer blend composition which exhibits PTC behavior; and comprises two crystalline, homologous polymeric materials, one of which has an MFI value of approximately 3 to 15 and constitutes approximately 5 to 45 percent by weight of the composition, while the other of which has an MFI value less than approximately 1 and constitutes approximately 2 to 40 percent by weight of the composition; and, preferably, comprises two types of carbon blacks, one of which has an average particle size of approximately 60 to 120 nm and a DBP value of approximately 60 to 200 cc/100 g and constitutes approximately between 45 and 56 percent by weight of the composition, while the other has an average particle size of approximately 10 to 20 nm and a DBP value of approximately 250 to 500 cc /100 g and constitutes approximately 2 to 10 percent by weight of the composition.
Alternatively, if the two polymeric materials of the above instance are nonhomologous in structure, then one of them should have an MFI value of approximately 5 to 1.5 and constitute approximately 5 to 45 percent by weight of the composition, while the other should have an MFI value less than approximately 1 and constitute approximately 2 to 40 percent by weight of the composition.
It is again understood that the total percentages of all the above constituents in (2) and (3) do not exceed 100 percent in each case.
In a further aspect the present invention provides an electrical device:
(1) comprising a conductive polymer composite which:
(a) exhibits PTC behavior;
(b) includes two crystalline polymeric materials, one of which has an MFI value of approximately 3 to 15 and constitutes approximately 5 to 45 percent by weight of the composite, while the other of which has an MFI value less than approximately 1 and constitutes approximately 2 to 40 percent by weight of the composite; and, preferably,
(c) comprises two types of carbon blacks, one of which has an average particle size of approximately 60 to 120 nm and a DBP value of approximately 60 to 200 cc/100 g and constitutes approximately 45 to 56 percent by weight of the composite, while the other has an average particle size of approximately 10 to 20 nm and a DBP value approximately 250 to 500 cc /100 g and constitutes approximately 2 to 10 percent by weight of the composite; it is understood that the total percentages of all the above constituents in (b) and (c) do not exceed 100 percent; and
(2) comprising two metal electrodes each of which:
(a) is at least 0.025 mm thick; and
(b) is in ohmic contact with said polymer composite and can be connected to an electrical power source to cause current to flow through said polymer composite.
Alternatively, if the two polymeric materials of (1)(b) above are homologous, then one of them should have an MFI value approximately 5 to 15 and constitute approximately 5 to 45 percent by weight of the composition, while the other should have an MFI value less than approximately 1 and constitute approximately 2 to 40 percent by weight of the composition.
An advantage of the present invention is that the resistivity of an electrical device comprising the conductive polymer blend composition of the present invention has a resistivity significantly lower than conventional ceramic-based thermistors.
Another advantage of the present invention is that an electrical device comprising the conductive polymer blend composition of the present invention has a greatly reduced post-resetting power dissipation.
A further advantage of the present invention is that an electrical device comprising the conductive polymer blend composition of the present invention can be easily manufactured.
Still another advantage of the present invention is that an electrical device having enhanced bonding strength between its electrodes and conductive polymer blend composite can be easily manufactured.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art after reading the following detailed description of the preferred embodiments which are illustrated in the several figures of the attached drawings.